JOURNAL OF THE BALKAN GEOPHYSICAL SOCIETY, Vol. 4, No 2, May 2001, p. 19-28, 8 figs. Study of the cross-border geothermal field in the Sarandoporos-Konitsa area by electrical soundings 1 1 1 2 2 3 H. Reci , G. N. Tsokas , C. B. Papazachos , C. Thanassoulas , R. Avxhiou and S. Bushati 1 Geophysical Laboratory, School of Geology, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece. 2 I.G.M.E.,Institute for Geology and Mineral Exploration, 70 Messogion Str., Athens. 3 Geophysical Center of Tirana, L.9. Blloku “Vasil Shanto”, Tirana, Albania (Received October 1999, accepted May 2000) Abstract: The geothermic field in Sarandaporos-Konitsa lies in the cross-border area between Albania and Greece. The field has several surface manifestations and extended geological investigations, including tectonic and geomorphological studies have been carried out. This work presents the field application and interpretation of vertical electrical sounding (VES) data in both sides of the borders. The purpose was to study the structure down to the depth of 1000 m, in a relative large area, where information about the deep structure would have an extreme cost if acquired by a network of boreholes. Two main geoelectrical formations that coincide with the flysch and the limestone basement are revealed. The area appears faulted in the NW-SE and NE-SW directions and a few concealed graben and horst structures exist. Low resistivity values were observed above basement uplifts and major faults. These values were attributed to hot fluid circulation. Key words: Electrical Sounding, Geothermal Field, Sarandaporos-Konitsa Area, Albania, Greece. INTRODUCTION present study. Further, quantitative geoelectrical models of the subsurface were produced along seven The studied area lies north of the city of Konitsa in sections by means of 1-D inversion of the VES data Greece and includes the Albanian village of Koukes using the steepest descend method (Koefoed, 1979). (Fig.1). Specifically, it lies between the geographic latitude (φ) 40o 02’ up to 40o 07’ and in geographic longitude (λ) 20 o 37’ up to 20 o 45’. GEOLOGICAL AND TECTONIC SETTING OF Figure 1 shows also the topography of the studied THE REGION area and the locations of VES measurements. The soundings are arranged along profiles ranging The area belongs to the External Hellenides, the approximately NE-SW. Thermal springs are present geological and tectonic zones which range parallel to both in the Greek and Albanian sides. Consequently, it the Adriatic Sea and cross both countries. In the is reasonable to assume that the same geothermal field Konitsa region, in particular, the geological formations extends in both sides of Greek –Albanian borders. belong to three different geotectonic zones; namely in Thermal springs known in the Greek part of the region the Ionian, Pindus and in the Subpelagonian zones are the thermal springs of Kavassila – Piksaria where (Koukuza and Perrier, 1963-1964) (Fig. 2). the water temperature reaches up to 31o C. In the The Ionian zone is assumed to be an indigenous Albanian side, the existence of hot springs (Koukes) is zone. Above its formations we have the thrusts of also well known, where the water supply is provided formations of the Pindus zone. The formations of by the hot springs situated in the Skordili bridge (Fig.1). Subpelagonian zone comprises fragmented tectonic The main target of the present study is the detection covering. This covering is situated above the of geothermal fluids, tectonic zones and faults using formations of the Pindus zone. In the tectonic context, electrical methods. Mapping of the relief of the the region presents a complicated picture of successive basement, which is considered as the ceiling of the Nappe and Flake tectonics, overlaid one above the limestone formation, comprises also a target of the other (Dimopoulos et al., 1990). © 2001 Balkan Geophysical Society, access http://www.BalkanGeophySoc.org 19 20 Reci et al. FIG. 1. The geographic position of the studied area. Hot springs are presented both in the village of Koukes (Albania) and in the location Kavasilon (Greece). The locations where VES’s were performed are marked by circles. These soundings were arranged along sections marked from T1 to T7 The tectonic structural characteristic of the Ionian Explorations (IGME), in two stages. During the first zone comprises of anticlinal and synclinal series with stage, in the spring of 1985, resistivity and self- NNW-SSE up to NW-SE directions, thrusted one potential (SP) measurements were carried out above the other towards the west. The main anticlinal (Thanasoulas, 1986a). The second stage took part structures in the region of interest are the Konitsa’s during the period June-July 1986 (Thanassoulas, anticline and the anticline of Skrodoli Bridge (Fig. 2). 1986b). In this stage, the investigations extended The main directions of faults are orthogonal and towards to the western side of the region which found intersecting each to the other. The primary fault to be of particular interest, after the preliminary system is of NNW–SSE direction (Dinaric direction). interpretation of the results of the first stage. The secondary fault system trends in the NE–SW A total number of 114 VES data measured in the direction. The Konitsa’s fault belongs to the later Greek part was used in the present study. These are system and it comprises an important component of arranged along the profiles T1 to T6, as shown in the the overall setting of the studied area. map of Figure 1. The soundings were spaced at about 200 m each from the other. Also, the results of 14 deep THE GEOELECTRICAL DATA soundings are used (Avxhiu, 1992), which were carried out in the Albanian side of the borders (Fig.1, Concerning the Greek side, the resistivity Fig.2). Some of these soundings are arranged along measurements were carried out in the area of Konitsa the profile T7 and they are not spaced at equal from the Institute of Geological and Mineral intervals. Study of the cross-border geothermal field by electrical soundings 21 INTERNAL AND CENTRAL Kavassila spring T1 IONIAN ZONE T7 ALBANIA T2 Upper Eocene - Aquitanian 40.10 T3 40.10 Flysch Undivided A Paleocene- Uper Eocene Sublithographic limestones T4 T5 OLONOS PINDOS ZONE Maestrichtian Upper Eocene T6 G R E E C E Flysch Upper Cretaceous Limestones 40.05 40.05 SUBPELAGONIC ZONE KONITSA Jurassic Ophiolitic Series Lowe Jurassic Limestones 40.00 40.00 20.63 20.66 20.69 20.72 20.75 1500 m 1000 m 0 m FIG. 2. Regional geological map of the studied area (Koukuza and Perrier, 1963-1964). The locations, where VES's have been conducted , are marked by dots All the resistivity measurements were carried out the re-digitised observed field curve. The program is using the Schlumberger array. The current line length based on the steepest descent optimisation method was 1600 m (AB/2=800 m) for the majority of the (Koefoed, 1979). At first it constructs a curve which soundings. Nevertheless, for a small number of VES, corresponds to the given model through the solution of located mainly in the centre of the Konitsa’s basin, the the direct problem by means of Ghosh’s filters current line length was 4000 m (AB/2= 2000 m). (Ghosh, 1971). Next, a measure of the misfit is assessed. The sum of the squared deviations between DATA PROCESSING AND QUALITATIVE the two types of the data is considered in this case. INTERPRETAION Then, the model parameters are changed successively, until the misfit measure falls below a predefined value. The analysis of VES data was done along the The minimisation of the misfit measure is achieved baselines described below. At the beginning, an through the steepest descent method. Thus, the refined approximate model was assessed to each sounding model parameters that give a misfit below the curve either guessing the subsurface conditions or predefined level, are used in the next stages of the using the three layers master curves and the associated study. auxiliary ones (Keller and Friscknecht, 1970). The However, qualitative interpretation of the model parameters, consisting of layer resistivities and geoelectric measurements was first attempted. Thus, thicknesses, were fed into a computer program along maps of the spatial distribution of apparent resistivity 22 Reci et al. were constructed for various current line expansion B) In all maps, the distribution of apparent resisti- lengths, i.e. the quantity vity values ρα shows that high values (ρα > 100 Ohm.m) dominate the northeastern and eastern part of the ρ (AB/2) = F(x,y) α region. The limestone formation outcrops in these was mapped. Figures 3 and 4 show the spatial particular locations. distribution of apparent resistivity for different half- C) The locations where the apparent resistivity current line spacing (AB/2). All apparent resistivity values are less than 100 Ohm.m are those where the distribution maps refer to the same elevation plane, flysch has a considerable thickness. which is +405 m above the sea level. D) Zones of a high conductivity (1/ ρα) are detected The study of apparent resistivity distribution maps in the region where the flysch dominates and they are shows the following: arranged along the NW- SE direction. The existence of A) Generally, the direction of resistivity contours is these zones might be attributed to the circulation of NW- SE. This direction corresponds to the direction of fluids (cold or hot) in the tectonic faults of the flysch. the contact between flysch and limestone, which E) From the tectonic point of view, the main appears north of Konitsa (Fig.2). features that dominate the area are the alignments FIG. 3. Distribution of apparent resistivities for half current lines (AB/2) a) 250 m, b) 320 m, c) 400 m, d) 500 m.